A common method for making benzoic acid is by catalytically oxidizing toluene. The oxidation is typically carried out in benzoic acid solvent and is catalyzed by a Co/Mn/Br catalyst system where the bromide acts as a catalyst promoter. Acetic acid can be used as solvent, but benzoic acid is preferred in order to simplify purifying the product. The oxidation is generally performed in a continuous process where toluene is fed to a reaction zone, and the product is removed at the same rate and purified by distillation.
The corrosive nature of the bromide promoter, however, restricts the materials of construction of the oxidation reactor to more expensive materials, such as titanium or nickel alloys. Less expensive materials, such as 304-stainless steel, have been avoided because bromide salts can readily corrode them. Thus, a catalyst system that does not employ bromide is of great value in that it would allow the construction of a manufacturing plant at a much lower cost, and would even allow the use of general, multi-purpose equipment.
It is well known that toluene can be smoothly oxidized to benzoic acid in acetic acid solution using an oxidation catalyst composed of only cobalt salts without the need for a bromide promoter. The co-oxidation of acetaldehyde and the presence of a zirconium co-catalyst are also known to enhance the rate of conversion. However, when the cobalt-catalyzed oxidation of toluene is carried out in benzoic acid solvent in the absence of a halide promoter, the rate of the oxidation is too slow to be of practical use. The slow rate is observed even in the presence of a zirconium promoter.
Thus, there is a need for a catalyst system and method for oxidizing toluene to benzoic acid in benzoic acid solvent that is free of halide, especially one that has a high enough oxidation rate to be of practical use.
The present invention addresses this need as well as others, which will become apparent from the following description and the appended claims.